Code phase signals and carrier phase signals in a Satellite Positioning System (SATPS), such as GPS or GLONASS, are subject to multipath signal errors. A code phase signal for GPS has an associated wavelength of 300 meters and 30 meters for C/A code and P code, respectively, and a typical multipath-based code phase location error will be about 1-10 percent of this wavelength. Spatially separating two SATPS antennas, which is usually done in differential SATPS operations, will often subject the signals received at each of these two antennas to multipath perturbations with different amplitudes and different phases.
Code phase signals are used in SATPS applications to provide a reasonably accurate measurement of a vector separating two SATPS antennas. Random noise in a code phase signal typically produces a location error of about .+-.(5-100) cm with zero mean for a one-second signal time average and is produced by thermal and atmospheric fluctuations in the signal path. With multipath signals absent, thermal and atmospheric fluctuations and atmospheric and antenna phase center delays would be the limiting factors on accuracy. Multipath signal error is thus an important factor in precise determination of a vector separating two SATPS antennas.
The intensity, phase and period of the multipath signals depends upon the physical environment in which the SATPS antenna operates. An environment in which relatively large multipath signals enter the antenna will cause correspondingly large code phase multipath signal errors. The multipath phase change, produce by a multipath signal that adds to or subtracts from the direct, undistorted signal, depends upon the relative rates of motion of the antenna and reflecting surface(s), the physical nature of the reflecting surface(s) and the satellite motion, among other things.
Previous attempts to mitigate, or compensate for, the effects of multipath signals on the ability of a two-receiver system to provide a precise measure of the separation vector have involved several approaches. In a first approach, the receiving antenna pattern is altered so that signal reflections from objects located at or near ground level are reduced. This approach usually requires use of physically large, and a fortiori non-portable, antennas and offers no mitigation for multipath signals received from reflecting surfaces that are located high off the ground. A second approach is to perform signal time averaging to reduce the multipath effects, with typical averaging times of 600-900 sec to significantly reduce multipath signal errors. This approach cannot be used for dynamic determination of the separation vector with a moving antenna.
Winters, in U.S. Pat. No. 4,007,330, discloses formation in parallel of three correlation functions, using a single locally general reference digital signal and three time-shifted replicas of a received digital signal with different time shifts. If the received signal has a nominal bit temporal length of .DELTA.t(bit), the three time shift delays are chosen to be .tau.1&lt;.DELTA.t(bit), .tau.2=.DELTA.t(bit) and .tau.3&gt;.DELTA.t(bit). The temporal bit length is contracted or expanded to account for expected Doppler shift of the received signal. The correlation signal with the highest value is chosen as the signal with the most likely time delay for the received signal.
A system for synchronizing a locally generated digital signal with a received signal is disclosed in U.S. Pat. No. 4,168,529, issued to Tomlinson. The system forms several correlation functions with associated time shifts equal to different integral multiples of an expected bit temporal length and determines the time shift corresponding to the correlation function with the largest value. The length of each sequence used to compute the correlation function is limited by the possible presence of Doppler shift, which is not accounted for directly in the choice of bit temporal length.
A pseudo-random number detection and tracking system using closed loop tracking is disclosed by Bowles et al in U.S. Pat. Nos. 4,203,070 and 4,203,071. The system uses a locally generated reference signal and a time-shifted received signal to form a non-linear response characteristic, usually a correlation function, and an associated error signal. A characteristic, such as time shift, of the correlation function is varied to minimize the error signal magnitude. Detecting the presence of an expected received signal requires a relatively large time shift range, and tracking of a received signal that is already detected requires a smaller time shift range.
Guinon et al, in U.S. Pat. No. 4,550,414, disclose an adaptive code tracker that receives a composite signal, which may include a direct signal and interfering signals, and forms correlation functions using the received signal and a plurality of time-shifted replicas of a reference signal. Each correlation signal is weighted, initially with uniform weights, and the weights are dynamically adjusted to vary and identify the time shift that produces the largest correlator function. The time shift increments can be fractions of a chip temporal width for the incoming signal.
In U.S. Pat. No. 4,608,569, Dickey et al disclose an adaptive signal processor for cancelling signal interference. A first time correlation function is performed, using a signal received by a highly directional antenna and one or more auxiliary interfering signals received by one or more adjacent omnidirectional antennas. The auxiliary signals are time-shifted and weights are computed and assigned to these signals. A sum of the weighted auxiliary signals is then subtracted from the main signal to produce a resultant signal that is freer from interference. Multiple signal arrival times are asserted to be accounted for with this approach.
Liebowitz discloses a multiplexed digital signal correlator in U.S. Pat. No. 4,660,164. A serial data stream is divided by a multiplexer into several parallel channels, in each of which a correlation function is computed using a locally generated reference digital signal and using the same time shift. The correlation function signals formed in the channels are combined to form an overall correlation function. The required signal processing rate for each parallel channel is less than the signal processing rate that would be required using a single channel.
A phase-coherent TDMA quadrature receiver for identifying arrival of a direct signal contained in a composite QPSK signal is disclosed in U.S. Pat. No. 4,829,543, issued to Borth et al. First and second correlation functions are formed, using a time synchronizing sequence and this sequence shifted by a quarter cycle, together with the received composite signal. The peak amplitudes of the two correlation functions are identified and used to determine a phase angle associated with the two correlation functions, from which a direct signal arrival time may be estimated.
McIntosh discloses logical ORing of a data signal with a selected pseudo-noise signal before transmission, using Manchester or differential encoding, in U.S. Pat. No. 4,862,478. The received signal is mixed with a first signal, related to the pseudo-noise signal, and with a time-delayed first signal and passed through a low pass filter. This approach assertedly produces received signals that are resistant to corruption by multipath signals.
U.S. Pat. No. 5,091,918, issued to Wales, discloses a signal equalizer that relies upon transmission with Gaussian Minimum Shift Keying of in-phase (I) and quadrature (Q) channel signals. A carrier wave of the received composite signal is removed, and the I and Q channel signals are analyzed to estimate the transmitted signals.
A multi-channel digital receiver for GPS signals is disclosed in U.S. Pat. No. 5,101,416, issued to Fenton. Correlation functions are formed, using the received signal and a selected pseudo-noise reference signal, and early, punctual and late correlation values are computed. The time delay spacings for the correlation values are dynamically adjusted to enhance the samplings of the received signal.
A digital equalizer system, using I and Q channel analysis of a received composite or distorted signal, is disclosed by Cai et al in U.S. Pat. No. 5,164,959. Correlation functions are formed using the received signal and a time-delayed reference signal in the I and Q channels. Using an estimated phase shift, signal energy in a dispersive channel (usually Q) is largely converted to signal energy in a non-dispersive channel before further analysis of the received composite signal.
Scott et al disclose a technique for correcting errors in sampling time and carrier frequency of a digital signal transmitted through a dispersive medium, in U.S. Pat. No. 5,282,228. First and second complex correlation functions are formed using the received composite signal, with phase components that provide estimates of the timing error and the carrier frequency error. These error estimates are used to adjust the complex phase and timing of subsequent received signals before the correlation functions are formed, to enhance the sampling of these signals.
A multipath noise reduction system for spread spectrum signals is disclosed by Meehan in U.S. Pat. No. 5,347,536. Early, prompt and late correlation values are computed for a correlation function formed using a time-shifted reference signal and the received signal, which may contain one or more multipath signals. In one embodiment, two early correlation values, computed at spaced apart time shift values, .tau.=t.sub.E1 and .tau.=t.sub.E2 &gt;t.sub.E1, and a prompt correlation value at a time shift value .tau.=tp are determined, and the multipath distortion is assumed to have the greatest effect at .tau.=tp and the least effect at .tau.=t.sub.E1. A slope of error signals is computed, using the correlation values at the time shift values .tau.=t.sub.E1, t.sub.E2, and this slope value is used to estimate the multipath-induced error at the prompt time shift value .tau.=tp.
These approaches do not determine multipath signal perturbations to a "clean" signal and do not provide real time capability for initially identifying and assisting lock-on to, or for re-locking to, a given SATPS satellite. What is needed is an approach that allows code phase multipath signal errors to be determined quickly and quantitatively, in a time interval of length considerably less than one see, that maintains portability of the SATPS ground equipment, and that is flexible enough to adapt to whatever is the present situation.